1. Field of the Invention
The present invention relates to a method of manufacturing a field emission cathode which emits electrons under the effect of a field applied between the cathode and a gate electrode.
2. Description of Related Art
The recent researches and development of display devices have been directed for thinner display structures. In these circumstances, the so-called field emission displays (will be referred to as "FED" hereinunder) having field emission cathodes disposed therein attract special attention from various industrial fields.
The FED is a flat CRT (cathode ray tube) of a field emission type having a field emission cathode, and an anode electrode and phosphors disposed opposite the field emission cathode in a position corresponding to each pixel. The pixels are disposed in the form of a matrix to thereby build a display.
In the FED, electrons emitted from the field emission cathode are accelerated by an electric field between the field emission cathode and anode electrode and impinge upon the phosphors which will thus be excited to emit light and display an image.
The field emission cathode used in the field emission type flat CRT of this type utilizes the tunnel effect of the electrons in a strong electric field. The electron-emitting materials include a high melting-point metals such as Mo, Ni, W, etc., and Si, etc. Many of the conventional cathode chip structures are of a so-called Spindt type.
To produce a Spindt type cathode chip, a base electrode formed from a conductive layer is first formed on a substrate of glass or the like, next an insulative layer is formed on the base electrode, a gate electrode formed from a conductive layer is formed on the insulative layer, and then fine holes (of 1 .mu.m in diameter) are formed in the gate electrode so as to reach the base electrode. Next, the above-mentioned high melting-point metal or Si is used to form a cathode chip in the hole. At this time, the lift-off technique is used to form a conical cathode-chip free end having a radius of curvature of a few tens of nm and directed towards the gate electrode.
The conical free end is less than 1 .mu.m high and the distance between the base and gate electrodes is less than 1 .mu.m with an insulative layer disposed between the electrodes. In the Spindt type cathode chip, when a positive electrode of a few tens of volts is applied to the gate and base electrodes, the conical free end of the cathode chip will have an electric field of about 10.sup.7 V/cm and emit a field of electrons.
In the field emission type flat CRT, the emitted electrons are made to impinge upon the phosphors on the anode electrode disposed opposite the conical free end of the cathode chip spaced 0.2 to 1 mm from the anode electrode. The phosphors will thus emit light.
Each of the pixels of the flat CRT consists of a few tens to a few thousands of Spindt type cathode chips. To build a display having a number of pixels (1024.times.768.times.RGB) in the XGA class being a standard specification of computer displays, for example, requires 0.1 to 100 billions of cathode chips.
The above-mentioned Spindt type field emission cathode is disadvantageous as will be described below:
Firstly, the Spindt type field emission cathode cannot be manufactured with a high yield and at a low manufacturing cost. More specifically, since the Spindt type field emission cathode has the aforementioned structure and works on the above-mentioned principle, the free end of the cathode chip is most important for concentration of electric field. For this field concentration, the free end has to be formed by the evaporation technique or the like to have a radius of curvature of a few tens of nm or less. Namely, since the working accuracy should be lower than a submicron order, similar process and equipment for manufacture of integrated circuits are required for production of the Spindt type field emission cathode. Therefore, when cathode chip group (cathode plate) is produced for a middle- to large-size screen, for example, 17 inches or more in diagonal dimension, an extremely large scale equipment and a vast plant and equipment investment are required, resulting in a considerably large increase of the manufacturing costs. Further, the cathode chips have to be produced evenly without any defect over the cathode plate surface. The larger the cathode plate size, the larger the number of cathode chips are required and the worse the yield becomes. Therefore, it is difficult to apply the Spindt type field emission cathode to a middle- to large-size screen in practice.
Secondly, the high melting-point metals Mo, Ni, W or the line and Si as the electron-emitting substances are weak against ion bombardment. They are easily deteriorated by the bombardment by the ions generated from the residual gas and phosphors. Thus, to ensure a long cathode life, the vacuum degree from this Spindt type field emission cathode must be one step or more lower than the vacuum degree for the ordinary CRT that is 10.sup.-6 to 10.sup.-7 Torr.
To solve this problem, the published document WO97/6549, for example, discloses a field emission plate or a flat CRT using the field emission plate, having a structure in which conductive particles are provided on a dielectric layer formed on a conductive layer provided on a substrate, a further dielectric layer is formed on the conductive particles and the thickness of each dielectric layer is 1/10 to 1/100 of the size of the conductive particle. The document also proposes a technique of producing the structure by printing or the like as a less expensive structure and manufacturing method suitable for manufacture of a large-screen flat display.
Further, the U.S. Pat. No. 5, 608,283 discloses a field emission cathode plate in which particles of graphite, amorphous carbon or silicon carbon are provided on high-resistance pillars formed on a conductive layer provided on a substrate or directly on the conductive layer via an adhesive layer.
In the method disclosed in WO97/6549, however, the conductive particles are provided on the conductive layer via the dielectric layer and the thickness of the dielectric layer has to be controlled to an order of a few hundreds of .ANG.. This control is very difficult. Therefore, this method is not suitable for manufacture of a large-screen cathode plate.
Also, the field emission cathode plate disclosed in the United States Patent is characterized in that the conductive particles are bonded to the conductive layer with a conductive adhesive. However, there is a large likelihood that the conductive adhesive material is likely to cover the conductive particles. In this case, electrons will not be emitted. To avoid this, it is necessary to control the thickness of the conductive adhesive to hundreds of .ANG.. However, this control is extremely difficult. Therefore, this method is not suitable for use to manufacture a large-screen cathode plate. Also it is difficult to dispose conductive particles selectively on the high-resistance pillars by the ordinary layer forming and printing techniques.